The proposed experiments are components of a research effort whose long-term objective is to understand the role played by the spinal cord in the control of voluntary movements of the primate arm. This goal includes a search for therapies or interventions that will overcome the motor deficits associated with central nervous system injury or disease. The proposed study advances these goals by elucidating the functional organization of spinal interneurons controlling forearm movements in the normal behaving monkey. A thorough understanding of normal spinal function is essential before studies of motor impairment and recovery following injury can be usefully interpreted. The specific aims of the project are: 1. How do descending and sensory signals interact at the level of individual spinal interneurons? 2. Do descending signals recruit subsets of functionally related spinal interneurons by activating them synchronously, and is this coupling of activity modified by sensory input? 3. Are individual motor units within a motor pool controlled independently by spinal interneurons and, if so, how are these interneurons selectively recruited by descending and afferent pathways? The activity of pairs of interneurons in the lower cervical spinal cord of monkeys will be recorded as they move their wrist and exert a power grip of the hand. Interneurons will be identified as having correlational linkages to arm muscles using spike-triggered averaging of electromyographic activity. The responses of interneurons to applied muscle stretch and cutaneous stimuli will be characterized when the muscles are relaxed, and during the preparation for and execution of movements. Cross-correlation histograms of the activity of pairs of interneurons will be calculated to detect synchronous firing in different phases of the movement. The activity of individual, type-identified motor units will be isolated and connections from interneurons detected with cross-correlations of their activity. Together, these studies will elucidate the role of interneuronal pathways in integrating descending and sensory signals to generate motor patterns and lay the foundation for future studies on the mechanisms of motor dysfunction and recovery from injury.